1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for handling an abnormal situation in a wireless communication system.
2. Related Art
Global system for mobile communication (GSM) is a radio technology which has been developed as a standardized system for radio communication systems in Europe and which has widely been deployed all over the world. General packet radio service (GPRS) was introduced to provide packet switched data services in circuit switched data services provided by the GSM. The enhanced data rate for GSM evolution (EDGE) employs an 8-phase shift keying (PSK) in addition to the Gaussian minimum shift keying (GMSK) employed in the GSM. The enhanced general packet radio service (EGPRS) represents the GPRS using the EDGE.
A packet data channel (PDCH) denotes a physical channel used for GPRS/EGPRS traffic. Examples of logical channels mapped onto the PDCH include a packet common control channel (PCCCH) used for control signals required in packet transmission initialization, a packet data traffic channel (PDTCH) for user data, a packet associated control channel (PACCH) for dedicated signaling, and so on.
An enhanced general packet radio service phase 2 (EGPRS2) supporting further various modulation and coding schemes has recently being developed. While the EGPRS supports only two modulation schemes (i.e., GMSK and 8-PSK), the EGPRS2 supports five modulation schemes (i.e., GMSK, quadrature phase shift keying (QPSK), 8-PSK, 16-quadrature amplitude modulation (QAM), and 32-QAM). There are two levels of EGPRS2, that is, EGPRS2-A and EGPRS2-B. The EGPRS2-A supports the GMSK, 8-PSK, 16-QAM, and 32-QAM. The EGPRS2-B supports the GMSK, QPSK, 16-QAM, and 32-QAM. A downlink EGPRS2-A uses modulation schemes MCS-1 to MCS-4 (MCS stands for Modulation and Coding Scheme) and DAS-5 to DAS-12 (DAS stands for Downlink level A modulation and coding Scheme). An uplink EGPRS2-A uses modulation schemes MCS-1 to MCS-6 and UAS-7 to UAS-11 (UAS stands for Uplink level A modulation and coding Scheme). A downlink EGPRS2-B uses modulation schemes MCS-1 to MCS-4 and DBS-5 to DBS-12 (DBS stands for Downlink level B modulation and coding Scheme). An uplink EGPRS2-B uses modulation schemes MCS-1 to MCS-4 and UBS-5 to UBS-12 (UBS stands for Uplink level B modulation and coding Scheme). A modulation and coding scheme for each level in the EGPRS2 may be found in clause 6.5.5.1.3 of 3GPP TS 43.064 V7.6.0 (2007-08) “Technical Specification; GSM/EDGE Radio. Access Network; General Packet Radio Service (GPRS); Overall description of the GPRS radio interface; Stage 2 (Release 7)”.
Hereinafter, an EGPRS system is a system to support EGPRS or both EGPRS and EGPRS2.
An EGPRS system provides a multi-data rate by using various modulation and coding schemes. For example, data is transmitted with various data rates through the PDTCH. The data rate is adjusted on the basis of the link quality in a link adaptation process. If the link quality is good, data is transmitted with a high data rate. On the contrary, if the link quality is poor, data is transmitted with a low data rate. Data may be lost when transmitted according to a modulation and coding scheme that requires a data rate higher than achievable with the link quality. In link adaptation, a data throughput is increased using the highest possible data rate by using a specific modulation and coding scheme with a predetermined link quality.
In the EGPRS system, link adaptation is performed by a radio link control (RLC)/medium access control (MAC) layer. The RLC layer and the MAC layer are respectively located in a mobile station (MS) and a base station (BS). Protocol data units (PDUs) of the RLC layer are delivered to an RLC entity, and are split in an RLC/MAC block unit by the RLC entity. Thereafter, communication is achieved in an RLC/MAC block unit. Each RLC/MAC block is numbered by a block sequence number (BSN). The BSN is used to modify an erroneous block by tracking an RLC/MAC block between a receiving RLC/MAC entity and a transmitting RLC/MAC entity. In downlink transmission, the BS requests the MS to send a status of a received block, and the MS transmits a packet downlink acknowledgment/negative-acknowledgment (ACK/NACK) message in response thereto. In uplink transmission, the MS requests the BS to send a status of a receive block, and the BS transmits a status report message (e.g., a packet uplink ACK/NACK message) in response thereto.
Processes used in the MAC/RLC layer are disclosed in 3rd Generation Partnership. Project (3GPP); Technical Specification Group. GSM/EDGE Radio Access Network; General Packet Radio Service (GPRS); Mobile. Station (MS)-Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol (Release 7), 3GPP TS 44.060 V8.1.0 (2008-05). Clauses 10.3a.3 and 10.3a.4 of the 3GPP TS 44.060 disclose an uplink RLC/MAC header and a downlink RLC/MAC header for various MCSs. Among fields included in the header, an uplink state flag (USF) indicates an owner or usage of a next uplink radio block in the same timeslot. A relative reserved block period (RRBP) field specifies a single uplink block in which the MS transmits a packet control acknowledgment message of a packet associated control channel (PACCH) block through a network. An EGPRS supplementary/polling (ES/P) field indicates an effectiveness or non-effectiveness of the RRBP field.
As a part of EGPRS evolution, latency reduction is proposed. The latency reduction includes two methods, i.e., reduced transmission time interval (RTTI) configuration and fast ACK/NACK reporting (FANR). According to the RTTI configuration, a radio block is transmitted through a PDCH-pair to reduce a period for transmitting one radio block to 10 ms from 20 ms which is required in the conventional basic transmission time interval (BTTI) configuration. According to the FANR, ACK/NACK information is piggy-backed on a radio block without having to transmitting ACK/NACK by using an additional message. Thus, the ACK/NACK can be transmitted and received without additional message assignment.
FANR activation is included in transmission when a downlink assignment message or an uplink assignment message is assigned to the MS by the network. When the FANR activation is instructed, the MS performs uplink transmission or downlink transmission by using the FANR.
According to clause 5.2 of the 3GPP TS 44.060, an MS supporting the latency reduction can be assigned with a temporary block flow (TBF) for which FANR is activated in the RTTI configuration or the BTTI configuration. If the MS is assigned with the TBF for which FANR is activated, the network activates FANR for all concurrent TBFs assigned to the MS. This implies that the FANR is activated or inactivated for the all concurrent TBFs. Further, the network individually instructs activation/inactivation of the FANR when the uplink TBF and the downlink TBF are concurrently allocated or reallocated.
At present, the MS applies the same FANR activation/inactivation to all concurrent TBFs. However, a method in which the MS handles an abnormal situation is not provided yet. Herein, the abnormal situation is that the network instructs inconsistent FANR activation/inactivation for the uplink TBF and the downlink TBF to the MS. That is, this is a case where the network instructs FANR activation to the MS as to the uplink TBF, and instructs FANR inactivation to the MS as to the downlink TBF. If the MS arbitrarily activates or inactivates the FANR for all concurrent TBFs, the network does not know how the FANR is operated by the MS, which may result in data loss in downlink transmission and/or uplink transmission.